A wind turbine is an energy conversion system which converts kinetic wind energy into electrical energy for utility power grids. Specifically, wind incident on blades of the wind turbine generator (WTG) causes a rotor of the WTG to rotate. The mechanical energy of the rotating rotor in turn is converted into electrical energy by an electrical generator. Because wind speed fluctuates, the force applied to the WTG blades and hence the rotational speed of the rotor/generator can vary. Power grids however require a constant frequency electrical power to be generated by the wind turbine.
One type of wind turbine that provides constant frequency electrical power is a fixed-speed wind turbine. This type of wind turbine requires a generator rotor that rotates at a constant speed. A disadvantage of such fixed-speed wind turbine is that it does not harness all of the wind's energy at different speeds and must be disabled at low wind speeds. Another type of wind turbine is a variable speed wind turbine. This type of wind turbine allows the generator to rotate at variable speeds to accommodate for fluctuating wind speeds. By varying the rotating speed of the generator rotor, energy conversion can be optimized over a broader range of wind speeds.
A full scale power converter based wind turbines (which is a type of variable speed wind turbine) usually includes a power converter having a generator side converter coupled to a grid side converter via a direct current (DC) link. The generator side converter regulates the power of the generator. This power passes through the DC-link, and is eventually fed to the grid through the grid side converter. The grid side converter regulates the grid side power. The same is true for the Doubly Fed Induction Generator (DFIG) systems where only a portion of the power from the generator passes through the power converter.
A wind turbine may be requested to shut down, for example due to a fault in the grid, component failure in the turbine/wind farm or requested by the grid operator. When the wind turbine reduces its power during shut down, the power produced by the wind turbine decreases to zero at a specified rate. The rate of decrease of power, commonly called the power ramp down rate, is dependent on the power ramp down rate of the generator. How fast the turbine power can decrease during a shut down is normally limited by the maximum ramp down rate of the generator e.g. 0.2 pu/sec.
However, certain grid codes/utilities may require the turbine to ramp down at a rate faster than the maximum ramp down rate of the turbine/generator e.g. 0.4 pu/sec. It is also foreseen that some countries may specify in their grid code requirements a minimum ramp down rate of turbines during shut down.
Hence it is desirable to provide a method of operating a wind turbine to provide a ramp down rate which is higher than the maximum ramp down rate of the turbine or generator.